1. Field of the Invention
The present invention relates to a characteristic amount calculating device for soldering inspection for calculating a characteristic amount for use in generation of inspection data to be used in an optical or X-ray transmissive appearance inspecting machine for inspecting a soldered portion of a component mounted on a printed wiring board.
2. Description of the Related Art
The inspection of a soldered portion by the optical or X-ray transmissive appearance inspecting machine is performed by inspecting the shape of a solder bonding a land on a printed wiring board and a component to be mounted on the printed wiring board to thereby inspect the connected condition of the solder for the purpose of ensuring long-term reliability. Both in the optical appearance inspecting machine and in the X-ray transmissive appearance inspecting machine, an inspection image indicating the characteristic of the three-dimensional solder shape is picked up.
In the case of the optical appearance inspecting machine, light is directed onto the solder from the upper side of the printed wiring board and reflected light from the surface of the solder is picked up. The solder surface has a capability of specular reflection, so that light incident on the solder surface in a specific direction is reflected on the solder surface in a specific direction. Accordingly, the inspection image indicates the surface angle of the solder shape. In the case of the X-ray transmissive appearance inspecting machine, X rays are directed to the solder from the upper or lower side of the printed wiring board to pick up (intensity detect) transmitted X rays. The transmittance of the X rays continuously changes with the thickness of the solder, so that the inspection image indicates the thickness of the solder.
In general, the inspection image indicating the characteristic of the three-dimensional solder shape reflects a difference in solder shape between a defective and a nondefective, and therefore has different characteristics between the defective and the nondefective. Accordingly, the inspection is performed by measuring different characteristic amounts between the defective and the nondefective and providing a threshold therebetween to perform defective/nondefective determination. The characteristic amount means an intensity average in an arbitrary region of the inspection image or the length or area of a region having an arbitrary intensity. There is a large difference in characteristic amount between the defective and the nondefective in terms of a general solder shape. Accordingly, by setting the threshold between the characteristic amount of the defective and the characteristic amount of the nondefective, the inspection can be performed. However, there are variations in solder shape generated, so that the characteristic amount of the defective may be similar to the characteristic amount of the nondefective in some case.
In this case, there is a possibility of “overtight” determination such that an actual nondefective solder shape is erroneously determined as defective or “undertight” determination such that an actual defective solder shape is erroneously determined as nondefective, depending upon the threshold set above. The “overtight” determination causes an increase in number of times of visual inspection to be performed in the subsequent step, and the “undertight” determination further causes a reduction in nonadjusted ratio in the subsequent step, causing an increase in cost of the subsequent step. It is therefore desirable to minimize the “overtight” determination and the “undertight” determination by adjustment of the threshold or adjustment of inspection data including modification of an inspection region or modification of an inspection method.
The adjustment of inspection data must be performed as checking the circumstances of occurrence of the “undertight” or “overtight” determination for all the defective and nondefective solder shapes, because there is a case that an excess reduction in the “undertight” determination may cause an increase in the “overtight” determination, or there is a case that an excess reduction in the “overtight” determination may cause an increase in the “undertight” determination. Conventionally, the adjustment of inspection data is performed as checking the circumstances of occurrence of the “undertight” or “overtight” determination for solder shapes formed in the past, by collecting past characteristic amounts, inspection images, and results of defective/nondefective determination in visual inspection. In this case, the information on the characteristic amounts collected over a long period of time includes information on every solder shape, so that the adjustment of optimum inspection data can be performed.
However, the conventional method has three problems. The first problem is that long-term data collection related to solder shapes must be carried out. In the conventional method, the information on every solder shape is obtained by data collection. However, since the frequency of occurrence of solder shapes causing the “undertight” or “overtight” determination is low and these solder shapes are various, the data collection must be made for a long period of time, resulting in the requirement of much time for optimization of the inspection data.
The second problem is that the defective/nondefective determination is ambiguous. The “overtight” or “undertight” determination is made by the comparison of the determination result by the visual inspection in the subsequent step and the determination result by the appearance inspecting machine. That is, the determination result by the appearance inspecting machine is evaluated under the condition that the determination result by the visual inspection is correct. However, the visual inspection is made by human sensory inspection with reference to an inspection standard on a solder amount and solder wettability. As a result, the defective/nondefective determination in the visual inspection is ambiguous, causing an interference with the optimization of inspection data. For example, there is a case that an actual defective may be determined as a nondefective in the visual inspection, causing the occurrence of false “overtight” determination. When data adjustment is made against this occurrence, “undertight” determination actually occurs rather than reducing the “overtight” determination.
The third problem is that there are variations in inspection image due to any factors other than solder shapes. The adjustment of inspection data should be made for a solder shape determining a defective or a nondefective. However, an inspection image and a characteristic amount are not in one-to-one correspondence to a solder shape, but vary due to any factors such as a solder surface condition other than solder shapes. As a result, there is a case that an extremely rare inspection image may be produced outside the range of normal variations due to solder shapes. If the inspection data is adjusted for such a rare inspection image, there is a possibility that the “undertight” or “overtight” determination may increase. Therefore, such a rare inspection image must be removed from the object to data adjustment. However, it is difficult to distinguish between such a rare inspection image and a normal inspection image, thus causing a difficulty of optimization of inspection data.